Known high-voltage cable end terminations and cable junctions typically comprise a rigid core insulator and an electrically insulating, elastomeric stress relief element coaxially arranged around a longitudinal axis and matching the rigid core insulator through a conical interface and an axially aligned current path. The current path connects a cable conductor encased in an insulation of the cable to a high-voltage current terminal arranged within the rigid core insulator. EP0731994B2 is a representative of such prior art. During operation of a high-voltage component the current path in general continuously conducts a rating current.
The rigid core insulator can be formed as a fine graded condenser core with a number of concentric electrically conductive field-grading layers arranged around the current conductor path and embedded in an insulating material, such as described in DE19945148A1. Said field grading layers are commonly produced in that at least two electrically conductive aluminum field grading layers are inserted in between neighboring spacer layers during the winding process of the condenser core. In particular, the core insulator can be produced using resin-impregnated paper technology in which crepe insulating paper is used as the spacer sheet material. After completing the winding process, the condenser body is impregnated by an epoxy resin followed by a hardening/curing process.
Moreover, the cable is fixed to a base part of the termination, for example a flange in case of a cable bushing, by a cable clamp and enters the interior of the termination through a stress relief element, e.g. a stress relief cone. The function of the stress relief element is to provide a smooth transition of the very high electric field in the electric insulation of the cable into a much lower electric field in the interior of the termination.
Basically, there are two interface design approaches known in the art. The first design approach resides on a so-called inner cone concept where the rigid core insulator has a conical portion that is directed radially inwards with respect to a longitudinal axis defined by the columnar overall shape of the core insulator. The stress relief element has a conical shell portion that is directed radially outwards matching the shape of the conical portion of the core insulator such that an interface is formed. The second design approach resides on a so-called outer cone concept where the core insulator has a conical portion that is directed radially outwards with respect to the longitudinal axis. The stress relief element has a conical shell portion that is directed radially inwards matching the shape of the conical portion of the core insulator such that an interface is formed again. EP0731994B2 is a representative of the second design approach. In both design approaches, the core insulator is typically comprising an epoxy-based resin or a similar non-conformable, rigid material, whereas the stress relief element is usually made of conformable, elastic elastomeric materials.
Both design approaches have in common that the mating quality of the interface needs to be superior such that no voids between the stress relief element and the conical portion of the core insulator are formed. This, because these voids are known to be responsible for causing a dielectric breakdown occurring along the interface between the rigid insulating element and the elastomeric stress relief element. In order to avoid forming voids at the interface large pressure is required to be exerted by the stress relief element against the surface of the core insulator. The pressure makes the elastic material of the stress relief element conform in such a way that the material fills all uneven imperfections of the surface of the core insulator. The pressure is also required at the interface between the inner surface of the stress relief element and the outer surface of the insulation of the cable entering the cable fitting.
The first design approach has the advantage over the second approach that it is easier to generate large and uniform pressure over both interfaces discussed above by using the stress relief element being made of soft elastomeric material and applying a force pressing the material into the space between the cable and the inner-cone opening in the core insulator. However, especially when a fine graded condenser core is used, the disadvantage resides in that the diameter of the high-voltage, typically innermost field grading layer has to be larger that the external diameter of the insulation of the cable making the overall diameter of the core insulator large. On the other hand, the second design approach leads to cable fittings having a smaller diameter of the core insulator compared to cable fittings of the first design approach designed for identical electric conditions because the second design approach allows for arranging the innermost field grading layer to be arranged more proximate to the conductor. The disadvantage of the second approach resides in that the required pressure exerted by the stress relief element on the core insulator is leads to comparatively bulky cable fittings having a comparatively large overall diameter.